This invention relates to a process and an apparatus for oxidizing or reducing a dissolved substance with a semiconductor photocatalyst, and more particularly to a process and an apparatus for oxidizing or reducing a dissolved substance, suitable for separation and recovery of metal ions by oxidation or reduction, synthesis or decomposition of a polymeric substance, or production of hydrogen by decomposition of water.
A known process for reducing a dissolved substance, for example, metal ions, in a solution, by using an action of semiconductor photocatalyst is disclosed in Japanese Patent Application Kokai (Laid-open) No. 60-50172. The object of the process is to conduct metal plating on the surface of a catalyst substrate provided on a thin layer of semiconductor substance; and thus, the oxidation or reduction reaction is limited to the surface of the substrate even if the oxidation or reduction is directed to a dissolved substance in a solution. Therefore, the reaction efficiency seems to be rather low. In the case of oxidation or reduction of metal ions in a system containing many ion species, such as solution resulting from spent nuclear fuel reprocessing, there exists another problem, i.e., that the reduction efficiency is considerably lowered by formation of reaction-inhibiting substances by irradiation with an exciting beam, for example, by formation of nitrous acid through decomposition of nitric acid.
Heretofore known examples of using fine particles of semiconductor photocatalyst have been limited only to decomposition of water to produce hydrogen by using a solar beam energy, as given in Japanese Patent Applications Kokai (Laid-open) Nos. 57-67002 and 59-203701. Now, extensive studies have been made on the utilization of semiconductor photocatalysts in other fields. In heretofore known examples, the exciting beams for irradiation are selected on the basis of the band gap energy of a semiconductor substance to be used without taking peculiar light-absorbing characteristics of the fine particles into account. In other words, the heretofore proposed processes have a rather low photo energy utilization.
In reprocessing spent nuclear fuel discharged from atomic power plants, on the other hand, adjustment of the valency of metal ions is an important step for separating the nuclear fuel substances and various fission products dissolved in the nitric acid solution. For example, in separation of such nuclear fuel substances as U and Pu from each other, it is necessary to adjust the valency of Pu to a trivalent state in which Pu is never extracted into the organic solvent, and in this connection the tetravalent U is generally used as a reducing agent. However, more than the stoichiometrical amount of U is required. This leads to a new problem that the isotope composition of U product will be deteriorated. Particularly in case of the increased amount of Pu as contained in the spent nuclear fuel from a fast breader reactor, the problem is quite considerable.
A reduction process by electrolysis is disclosed in Japanese Patent Application Kokai (Laid-open) No. 48-23689, but the voltage loss through a solution is so large that an overvoltage is required, resulting in a poor energy efficiency. Also, an apparatus for the process will be complicated and will have a problem in its maintenance.
On the other hand, recovery of noble metals from a high level reprocessing waste solution has been regarded as important from the viwpoint of recovery of valuable metals, and a process disclosed in Nuclear technology, 65, page 305 (1984) seems to be promising, but owing to use of such a high temperature as 1,200.degree. C., it still has such problems as volatilization of ruthenium contained in the waste solution and resulting material corrosion, etc. Similar problems are also encountered in a step of heating a waste solution containing Ru, that is, in a concentrating apparatus for the waste solution and an evaporating apparatus for the acid recovery, and it is desirable to remove Ru from the waste solution before the treatment, but no effective process for removing Ru has been available yet.